Mechanical components are often designed to meet specific requirements for hardness, strength, material composition, and surface condition. These properties help determine whether a component will perform as expected in its final application.
During production, however, variations in heat treatment, material condition, or other manufacturing processes can cause these properties to change.
Manufacturers therefore need reliable ways to verify that components have been hardened correctly without slowing production or damaging the parts being inspected.
Eddy current testing provides a fast, non-destructive method for evaluating hardness-related material properties in ferromagnetic components. However, residual magnetism can interfere with the inspection and lead to unreliable results.
Demagnetizing components before testing can help reduce this interference and improve the repeatability of the hardness inspection.
Why Is Hardness Testing Important for Mechanical Components?
Hardness is an important material property for many mechanical components.
Manufacturers use processes such as induction hardening and furnace hardening to create the material characteristics required for a particular application.
For the process to remain consistent, parameters such as temperature, heating time, cooling conditions, and material composition must be controlled.
If these conditions change, the resulting hardness can also change.
This may lead to:
- Incorrect surface hardness
- Incorrect hardening depth
- Greater variation between components
- Components outside the required specification
- Increased scrap or rework
Hardness testing helps manufacturers identify these variations before components continue through production or enter service.
What Methods Are Used to Test Hardness?
Several established methods can be used to determine the hardness of metallic components.
Common methods include:
These methods provide quantitative hardness measurements, but they are not always ideal for inspecting every component in high-volume production.
Depending on the method and application, conventional hardness measurement may require contact with the component, leave an indentation, or require more time than is available within the production cycle.
When manufacturers need to inspect large quantities of components quickly and without damaging them, eddy current-based material testing can provide an alternative comparative approach.
Can Eddy Current Testing Be Used to Test Hardness?
Eddy current testing can be used to evaluate material properties that correlate with hardness in ferromagnetic components.
Every ferromagnetic material has characteristic magnetic properties. These properties can change when the material structure and hardness change.
An eddy current testing system applies an alternating magnetic field to the component and evaluates the resulting electromagnetic response.
When known reference components are available, the system can establish a relationship between their magnetic response and known material conditions.
Production components can then be compared against these reference values to determine whether they fall within the established acceptable range.
This is an important distinction: eddy current testing generally provides a comparative evaluation of material condition rather than a direct numerical hardness measurement.
Why Is Eddy Current Testing Useful in Automated Hardness Inspection?
One of the primary advantages of eddy current testing is that the inspection is non-destructive.
Components can be evaluated without creating the indentation associated with some conventional hardness measurement methods.
The technology can also operate at very short cycle times, making it suitable for integration into automated production.
Depending on the application, eddy current hardness testing can support:
- High-speed inspection
- Non-destructive testing
- Automated OK/NOK sorting
- 100% production testing
- Reduced dependence on manual inspection
- Repeatable comparison against reference parts
However, reliable results depend on maintaining consistent testing conditions.
What Can Affect Eddy Current Hardness Testing Results?
Several factors can influence the electromagnetic signal used to evaluate a component.
Important considerations include:
- Component position
- Test speed
- Probe or coil position
- Material composition
- Temperature
- Test frequency
- Residual magnetism
A reproducible inspection requires these conditions to remain as consistent as possible.
Residual magnetism is particularly important when testing ferromagnetic components because it can directly influence the magnetic response measured by the eddy current system.
What Is Residual Magnetism?
Ferromagnetic components can retain a magnetic field after being exposed to magnetizing influences.
This remaining magnetic field is known as residual magnetism or remanence.
Components can become magnetized during manufacturing or handling through contact with equipment such as:
- Magnetic lifting devices
- Magnetic grippers
- Magnetic conveyors
- Other magnetized tools or machinery
The amount and orientation of residual magnetism can vary from one component to another.
This becomes a problem when the same components are later inspected using an electromagnetic testing method.
Why Does Residual Magnetism Affect Eddy Current Hardness Testing?
Eddy current hardness testing evaluates the electrical and magnetic response of a component.
If the component already contains residual magnetic fields, those fields can interact with the magnetic field generated during the inspection.
The resulting test signal may therefore be influenced by remanence rather than only by the material condition the system is intended to evaluate.
This can increase signal variation and make it more difficult to reliably separate acceptable and unacceptable components.
In some cases, the altered signal may exceed the established sorting threshold even though the component meets the required material specifications.
The result is a false reject, sometimes referred to as pseudo rejection or pseudo scrap.
How Does Demagnetization Reduce False Rejects?
Demagnetization reduces residual magnetic fields before the component enters the eddy current testing system.
By reducing this additional magnetic influence, components begin the inspection from a more consistent magnetic condition.
This can help:
- Reduce variation in test signals
- Improve inspection repeatability
- Reduce false rejects
- Improve OK/NOK classification
- Create more stable testing conditions
The objective is not necessarily to eliminate every measurable magnetic field from the component.
Instead, the remaining field should be reduced enough that it no longer significantly interferes with the hardness inspection.
How Are Components Demagnetized Before Hardness Testing?
One common approach is to expose the component to an alternating magnetic field with decreasing amplitude.
The changing field repeatedly reverses the magnetic orientation within the material while gradually reducing its strength.
As the magnetic field decreases, the residual magnetism in the component is reduced.
The process continues until the remaining magnetic field is low enough that it does not significantly influence the subsequent eddy current inspection.
For many applications, reducing the residual field to a level comparable with the Earth's magnetic field may be sufficient.
The required residual field level, however, depends on the component and inspection requirements.
Why Can Some Components Be Difficult to Demagnetize?
Demagnetization is not equally simple for every component.
Several factors can affect how easily residual magnetism can be reduced, including:
- Material properties
- Component geometry
- Size
- Orientation of the magnetic field
- Strength of the existing remanence
Some components may contain localized magnetic areas or magnetic fields acting along different axes.
In these cases, changing the orientation of the component relative to the demagnetization field can improve the result.
This can be challenging in automated production because the component usually moves through the process in a fixed orientation.
Materials with particularly high remanence may also retain measurable residual fields even after an intensive demagnetization process.
How Can You Check Whether a Component Is Properly Demagnetized?
When residual magnetism is a concern, magnetic field probes can be used to check the component after demagnetization.
This provides a separate verification that the remaining magnetic field is within an acceptable range for the inspection process.
If hardness test results show unexpected scatter or an increase in false rejects, checking residual magnetism can also help determine whether magnetization is contributing to the problem.
Questions to consider include:
- Are test results becoming less repeatable?
- Are known good components being rejected?
- Have magnetic handling devices been introduced into production?
- Are different components showing different levels of remanence?
- Does the test signal become more stable after demagnetization?
These checks can help separate a true material or hardness problem from magnetic interference.
When Should Demagnetization Be Added to a Production Line?
Demagnetization should be considered when residual magnetic fields can interfere with an electromagnetic inspection.
For eddy current hardness testing, positioning the demagnetization process before the inspection station helps establish a more consistent magnetic condition for each component.
This can be particularly useful in automated production environments where:
- Components are handled with magnetic equipment
- Large quantities of parts are inspected
- Test repeatability is critical
- Automated sorting is used
- False rejection rates need to be minimized
The demagnetization process should be developed around the material, component geometry, cycle time, and required residual field level.
How Can Demagnetization Improve Automated Hardness Testing?
Automated hardness inspection depends on consistent testing conditions.
If residual magnetic fields vary from component to component, the eddy current system may see additional signal variation unrelated to the actual hardness condition.
Demagnetization helps remove one of these variables before testing.
When properly integrated into the production process, it can contribute to:
- More repeatable inspection signals
- More reliable sorting decisions
- Reduced pseudo rejection
- Improved process stability
- Greater confidence in automated hardness testing
The result is a more controlled inspection process where changes in the test signal are more likely to represent actual differences in the material.
What Demagnetization Solutions Are Available for Automated Production?
For automated production environments, the demagnetization system needs to provide sufficient field strength while also meeting the required production cycle time.
Cestriom and FOERSTER developed the ZMAG demagnetization unit for the demagnetization of components in series production.
The appropriate demagnetization setup depends on factors such as:
- Component material
- Component size and geometry
- Initial level of magnetization
- Required residual magnetic field
- Component orientation
- Production cycle time
- Eddy current inspection requirements
Evaluating these conditions helps determine how demagnetization should be incorporated into the overall testing process.
Is Demagnetization Necessary for Your Hardness Testing Process?
If you are using eddy current testing to evaluate hardness or heat treatment condition in ferromagnetic components, residual magnetism should be considered when developing the inspection.
Not every application will require the same level of demagnetization. However, if residual magnetic fields are causing signal variation, inconsistent results, or false rejects, demagnetizing the components before testing can help create a more stable inspection process.
The key is to determine whether the variation in the test signal represents an actual material difference or an external magnetic influence.
Experiencing inconsistent hardness testing results or unnecessary rejects?
